A nightingale may – or indeed may not – have sung in Berkeley Square, but its song can now no longer be heard in many of its former haunts. According to the latest figures from the British Trust for Ornithology, Britain's nightingale population has plunged by more than 90% in just 40 years.

Nightingales have never been common in Britain. They are here for just a few months, and only sing for a few weeks, from late April to early June. They are also confined to southern Britain, roughly south and east of a line between the Severn and the Wash, with strongholds in East Anglia and Kent.

But now numbers are falling even here. Nightingales are birds of the undergrowth, nesting in dense, inaccessible scrub. This is one reason why their song is so celebrated – for actually seeing this skulking songster is often virtually impossible. Recently, however, this habitat has been under threat from the boom in deer numbers – especially the alien incomer, the muntjac. Muntjacs feed by browsing – and their insatiable appetite has destroyed much of the nightingale's scrubby home.

The nightingale's problems don't stop there. Like other migrant birds that spend the winter in west Africa, its numbers have been dropping like a stone – probably due to the southward extension of the Sahara desert.

If you want to hear the nightingale's song – and, believe me, it's worth it – you're too late this year. I'd advise putting it in next year's diary – it may be one of the last springs this extraordinary sound can still be heard in the British countryside.

Stephen Moss is a naturalist, writer and broadcaster, based at the BBC Natural History Unit.